1. Field of the Invention
The invention relates to a multilayer material for sliding elements which comprises at least a backing member and an overlay comprising electrodeposited tin-based metal matrix material, especially of SnCuNi, PbSnCu, PbSn, Sn, SnCu or CuSn, hard particles being embedded in the matrix material. The invention also relates to a process for producing such sliding elements.
2. Description of Related Art
Sliding elements include all types of bearing shells as well, inter alia, as pistons and piston rings. Overlays of multilayer materials for sliding elements are generally produced by electroplating in appropriate electrolyte baths.
It is known moreover, for example from "Oberflachen- und Dunnschichttechnologie" ("Surface and thin-layer technology"), part 1, Plating of Surfaces, by Rene A. Hafer, Springer-Verlag 1987, pages 198 to 200, to modify the characteristics of electrodeposited metal layers for example by adding hard materials or sliding materials to the electrolytes in the form of powdery particles and then depositing them on the substrate together with the metal. During electrolysis, the suspended particles migrate to the cathode, where they are incorporated in the metal being deposited. In this connection, it is attempted to incorporate particles which are as small as possible into the metal matrix, because in this way dispersion hardening is achieved, whereby hardness, wear resistance and strength are improved above all at elevated temperatures.
However, these possibilities exist only in theory, because the surface energy of the particles is greater, the smaller the diameter of the particles. The consequence thereof is the formation of agglomerates in the electrolytes, wherein air pockets form between the particles which, upon incorporation of the agglomerates into the matrix, result in lattice defects and thus in brittleness of the overlay.
In "Developments in Tri Metal Bearings", paper 2 by T & N, 1995, this problem is discussed and a suggestion made of a promising process in the form of an agitation process with, which cluster formation could be limited to the size of from 2 to 3 .mu.m.
Fluoroborate baths have been used hitherto as the electrolyte, but have the disadvantage that the particles are only inadequately wetted, the consequence of which is that, even when a large number of hard particles are available in the electrolyte, only a limited amount, as a rule up to a maximum of 2 vol. %, may be incorporated into the matrix.
Moreover, overlays, especially ternary layers, produced with known baths sometimes exhibit considerable variations in thickness, which may possibly necessitate post-machining of the sliding elements. Furthermore, the tin is not evenly dispersed in the overlay, which may cause the formation of clusters and coarse crystalline deposits, known as tin agglomerations. This inhomogeneous structure of the overlay promotes diffusion, which occurs when the sliding element is exposed to relatively high heat levels during operation, such that such overlays may only be applied to an intermediate layer, such as a nickel barrier for example, which prevents tin diffusion into the lead-bronze layer thereunder, as described, for example, in E. Romer "Werkstoff und Schichtaufbau bei Gleitlagern" ("Material and Layer Structure in Plain Bearings"), an offprint from ZFW Zeitschrift fur Werkstofftechnik ("Journal of Materials Technology"), volume 4, issue 7, Verlag Chemie Weinheim/Bergstra.beta.e 1973. Only through this additional measure has it hitherto been possible to improve corrosion resistance and to prevent detachment phenomena in the overlay, which may lead to bearing damage. Moreover, the hardness of these known overlays and thus their wear resistance are inadequate.